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Bacteria's Sweet Deception

Our intestines are filled with billions of bacteria that somehow evade detection by the immune system. A new study suggests the microbes hide by coating themselves in sugars harvested from host cells--a finding that could help researchers understand how hosts tell friend from foe.

Bacteria are hugely successful at colonizing the gut, but it's a big mystery how they maintain cordial relations with their hosts. One long-standing theory holds that bacteria use “molecular mimicry,” presenting proteins and sugars on their surface similar to those on the surface of host cells.

A species common in the colon, Bacteroides fragilis coats its surface with a large variety of complex sugars, some of which it builds with the simple sugar L-fucose. The bacteria make L-fucose themselves and can also obtain it from intestinal cells. But researchers weren't clear if the bacteria used this intestinal sugar for protection or merely for fuel.

To investigate, microbiologist Laurie Comstock's group at Harvard Medical School in Boston, Massachusetts, knocked out B. fragilis's genes for making L-fucose. To their surprise, the bacteria kept pumping out all the usual surface sugars. They then found that the bacteria have a so-called salvage pathway that allows them to build complex sugars using L-fucose from the environment--a pathway known before only in mammals. Yet the sugar is clearly essential, Comstock's group reports in the 18 March issue of Science. The normal B. fragilis easily outcompetes a mutant strain lacking the pathway for making and scavenging L-fucose.

The study sheds new light on how gut bacteria camouflage themselves, says geneticist Jian Xu of Washington University School of Medicine in St. Louis, Missouri. But he notes that how exactly these sugars help the bacteria evade the immune system is still a mystery.